In recent years, considering environmental issues, attention has been paid to a vehicle employing an electric motor as a source of drive force, such as an electric vehicle, a hybrid vehicle, and a fuel cell vehicle. Such a vehicle is equipped with a power storage unit implemented for example by a rechargeable battery for supplying electric power to the electric motor and converting kinetic energy to electric energy during regenerative braking.
In such a vehicle employing an electric motor as a source of drive force, it is desirable to further increase the charge/discharge capacity of a power storage unit to improve acceleration performance and running performance such as a maximum traveling distance. As a method for increasing the charge/discharge capacity of a power storage unit, a configuration equipped with a plurality of power storage units has been proposed.
For example, U.S. Pat. No. 6,608,396 discloses an electric motor power management system providing a high-voltage vehicle traction system with a desired high direct current (DC) voltage level. The electric motor power management system includes a plurality of power stages connected in parallel and each having a battery and a boost/buck DC-DC converter for supplying DC power to at least one inverter, and a controller controlling the plurality of power stages such that the plurality of power stages can maintain a voltage output to the at least one inverter by uniformly charging/discharging the batteries of the plurality of power stages.
It is disclosed that, in the electric motor power management system, each battery is actively maintained to be in the same SOC (State of Charge) as that of other batteries in the system. However, batteries in the same SOC do not always have the same power storage voltage value (i.e., a voltage value at an open end), because the voltage value of each battery greatly varies depending on the temperature of the battery, the degree of deterioration, and the like, in addition to the SOC.
On the other hand, at system start-up or the like, buck switches 38 are all turned ON in FIG. 1 of U.S. Pat. No. 6,608,396, and thus the batteries are connected in parallel to an identical high voltage DC bus 48 (power line) through respective boost/buck DC-DC converters 13 (voltage conversion units). Therefore, when the batteries have different voltage values, a current corresponding to the voltage difference flows between the batteries, causing unwanted electric energy shift between the batteries and increasing loss.